On Tue, 2019-10-08 at 15:42 +0200, Petr Mladek wrote: > On Tue 2019-10-08 09:23:52, Qian Cai wrote: > > On Tue, 2019-10-08 at 09:13 -0400, Steven Rostedt wrote: > > > On Tue, 8 Oct 2019 10:15:10 +0200 > > > Petr Mladek <pmladek@xxxxxxxx> wrote: > > > > > > > There are basically three possibilities: > > > > > > > > 1. Do crazy exercises with locks all around the kernel to > > > > avoid the deadlocks. It is usually not worth it. And > > > > it is a "whack a mole" approach. > > > > > > > > 2. Use printk_deferred() in problematic code paths. It is > > > > a "whack a mole" approach as well. And we would end up > > > > with printk_deferred() used almost everywhere. > > > > > > > > 3. Always deffer the console handling in printk(). This would > > > > help also to avoid soft lockups. Several people pushed > > > > against this last few years because it might reduce > > > > the chance to see the message in case of system crash. > > > > > > > > As I said, there has finally been agreement to always do > > > > the offload few weeks ago. John Ogness is working on it. > > > > So we might have the systematic solution for these deadlocks > > > > rather sooner than later. > > > > > > Another solution is to add the printk_deferred() in these places that > > > cause lockdep splats, and when John's work is done, it would be easy to > > > grep for them and remove them as they would no longer be needed. > > > > > > This way we don't play whack-a-mole forever (only until we have a > > > proper solution) and everyone is happy that we no longer have these > > > false positive or I-don't-care lockdep splats which hide real lockdep > > > splats because lockdep shuts off as soon as it discovers its first > > > splat. > > > > I feel like that is what I trying to do, but there seems a lot of resistances > > with that approach where pragmatism met with perfectionism. > > No, the resistance was against complicated code changes (games with > locks) and against removing useful messages. Such changes might cause > more harm than good. I don't think there is "removing useful messages" in this patch. That one printk() in __offline_isolated_pages() basically as Michal mentioned it is that useful, but could be converted to printk_deferred() if anyone objected. It is more complicated to convert dump_page() to use printk_deferred(). > > I am not -mm maintainer so I could not guarantee that a patch > using printk_deferred() will get accepted. But it will have much > bigger chance than the original patch. > > Anyway, printk_deferred() is a lost war. It is temporary solution > for one particular scenario. But as you said, there might be many > others. The long term solution is the printk rework. > > Best Regards, > Petr
